These studies constitute the continuation of six years of intensive research on BK virus, a human papovavirus. They are concerned with the biology, molecular biology, and mechanism of oncogenicity of this virus, a potential human oncogenic virus. BK infects 100% of the human population by the age of 5, is a potent transforming agent in vitro, and induces tumors in experimental animals. Most of the investigations proposed in this project are aimed at the elucidation of the possible role of this virus in human neoplasia.